1. Field of the Invention
The present invention relates to an ink jet printing method for forming an image on a print medium by ejecting ink onto it. More specifically, it relates to a printing method for an ink jet printing apparatus that prints an image using a plurality of inks with different characteristics.
2. Description of the Related Art
An ink used in conventional ink jet printing apparatus is generally composed of water as a main component and a high boiling point water-miscible solvent, such as glycol, to prevent ink from drying and getting clogged. However, the use of such an ink in printing operation may cause problems of the ink being insufficiently fixed in a print medium or the ink being soaked into the medium unevenly in a certain direction due to an ununiform distribution of loading filler and sizing agent in the surface of the print medium, resulting in a degraded quality of printed images. Further, when a color image is printed using a plurality of color inks, another problem has been observed in which inks of different colors come into contact and mix with one another before being soaked into the medium, rendering boundary sections between different colors in a printed image blurred. In the following description, such an ink mixing or spreading at the boundary sections is referred to as “bleeding.”
To address these problems printing apparatus incorporating a fixing device are being provided. By passing a print medium through the fixing device immediately after printing, printed ink can be quickly dried and prevented from bleeding. The printing apparatus with a built-in fixing device, however, becomes large in size, increasing the manufacturing cost. Further, since two steps—printing and fixing—are required, an image output speed decreases.
In light of this problem, rather than incorporating a new device such as the fixing device, a new method is being adopted frequently which makes improvements on the ink itself to solve the fixing and bleeding problems. For example, Japanese Patent Laid-Open No. 55-66976 discloses an ink mainly composed of a volatile solvent. Since a volatile ink quickly dries upon landing on a print medium, bleeding does not easily occur. This ink, however, may cause a new problem of ink clogging because the volatile ink easily evaporates from ejection openings in a print head that accommodates the ink.
Japanese Patent Laid-Open No. 55-65269 discloses an ink containing a compound that enhances a penetration of ink, such as surfactant. An ink with high permeability can soak into a print medium quickly upon landing, thus preventing contact among inks of different colors on the print medium and making their bleeding less likely. Since this ink is not volatile as is the ink of Japanese Patent Laid-Open No. 55-66976, there is no fear of an ink clogging problem caused by evaporation. Therefore, a growing number of color ink jet printing apparatus in recent years are using an ink with high permeability, such as disclosed in Japanese Patent Laid-Open No. 55-65269, because it can be handled relatively easily while reliably alleviating the problem of fixing and bleeding.
However, since the ink with high permeability also has its colorant soak deep into the print medium, there is a drawback that an image density and color saturation on the print medium surface tend to be lower than those of the conventional inks. Further, since ink tends to penetrate along fibers of the print medium, line arts such as characters can lose sharpness, rendering a so-called feathering more noticeable. That is, an ink characteristic tending to increase a fixing performance and avoid bleeding and an ink characteristic tending to increase an image density and suppress feathering are incompatible with each other. That is, these two opposing characteristics are difficult to achieve at one time with one kind of ink.
In recent years therefore, a printing construction and a printing method have been proposed and implemented in which a plurality of inks with different characteristics are incorporated in the same printing apparatus, with ink permeability differentiated for instance between a black ink often used to print letters and color inks often used to print graphics and picture images.
Japanese Patent Laid-Open No. 64-63185, for example, discloses a technique in which an ink jet print head to eject a liquid compound for making a dye contained in ink insoluble is prepared and in which the compound and ink are mixed on a print medium. Further, Japanese Patent Laid-Open Nos. 9-020070 and 9-025442 disclose a method in which a component for insolubilizing a black ink is contained in a particular color ink and in which the color ink that reacts with the black ink is printed where the black ink is to be printed.
Further, Japanese Patent Laid-Open No. 6-87222 discloses a method that prints boundary sections between color images and black images with a black that is produced by mixing color inks, in order to avoid bleeding between a black ink with low permeability and color inks with high permeability.
Also available in the market in recent years is an ink jet printing apparatus that uses two kinds of black ink—a black ink to emphasize the print quality of black letters and a black ink to emphasize the print quality of smooth photographic images.
A black ink emphasizing the print quality of black letters often has a higher content of colorant than other colors or uses a pigment with higher coagulation characteristic as a colorant to realize a higher density. When such an ink is used with an ink jet print head having fine nozzles to eject as small an ink droplet as possible, the ink at a front end of the nozzle tends to increase its viscosity as a solvent evaporates. In descriptions that follow, ink whose viscosity has increased as the result of solvent evaporation is called a viscous ink. The viscous ink is more difficult to eject if applied the same energy and the direction and speed of ejected viscous ink droplets are unstable, often resulting in shifted dot landing positions and loss of dots on a print medium. It is noted, however, that since the ink viscosity begins to increase only after the evaporation from the nozzle openings has proceeded to some extent, the viscosity increase does not occur easily with the nozzles that continuously eject ink. It is also noted that if the ink becomes viscous, the ink ejection state often returns to normal after a few unstable ejections are performed. That is, a first ejection from a nozzle that has not performed an ejection operation for some time is influenced by an extent to which the ink becomes viscous. A stability of the first ink ejection from such a nozzle is referred to as first ejection stability in this specification.
It is true that even if the first ejection stability is bad, the quality of printed image can be kept normal to some extent by periodically making a preliminary ink ejection from a nozzle. However, performing the preliminary ejection requires moving a carriage mounting the print head to a position where the preliminary ejection can be done, i.e., to a predetermined position outside a print medium, which in turn takes an extra time in addition to a real printing time. Depending on a combination of ink and a high-precision print head of recent years or on a size of print medium, there are cases where a deteriorated first ejection stability is observed even during one printing scan of the carriage and the effect of the preliminary ejection does not last for one complete printing scan. Further, the frequent preliminary ejection is not desirable because many preliminary ejection operations can consume a considerable volume of ink for other than printing, increasing a running cost.